The invention relates to a measuring and damping resistor arrangement for a high-voltage apparatus, notably an X-ray generator, in which a measuring resistor arrangement carries a high voltage and is connected to a damping resistor arrangement on the high-voltage side in the operating condition.
FIG. 1 shows a known circuit arrangement of this kind in the form of an X-ray generator. The anode connection 2 of an X-ray tube 1 is connected to a positive high voltage while its cathode connection 3 is connected to a negative high voltage. The high voltages are supplied by a high-voltage generator 4. The terminal 2 which carries a positive high voltage with respect to ground is connected, via a damping resistor 7, to the output of a high-voltage rectifier 5 which is connected to a high-voltage transformer (not shown) and whose output voltage is smoothed by a capacitor 6. The damping resistor 7 serves to limit the current through the X-ray tube in the case of a tube breakdown.
The voltage across the X-ray tube can be measured by means of a measuring divider which is connected between the connection 2 and ground. The measuring divider consists of ten series-connected RC-members 10 . . . 19, only the first (10) and last (19) of which are shown and which are connected to ground via a terminating RC-member 8. The resistance of each of the RC-members 10 . . . 19 amounts to 10 Mohms, while the resistance of the RC-member 8 amounts to only 10 kohms, so that the high voltage is reduced by one tenth by each of the resistors of the RC-members 10 . . . 19 and one ten thousandth part of the high voltage is present across the resistor of the RC member 8 for the purpose of measurement.
The construction of the section of the high-voltage generator 4 which generates the negative high voltage for the connection 3 of the X-ray tube 1 is identical to that of the section described thus far; however, the polarity of the rectifier must be reversed in order to ensure that the high voltage generated has the reverse polarity.
It is essential that the voltage on the terminating member 8 follows quick variations on the connection 2, i.e. the transfer bandwidth of the measuring divider 8, 10 . . . 19 must be sufficiently large. To this end, parallel-connected capacitors are provided in the individual RC-members; these capacitors are proportioned so that the same time constant (approximately 10 ms) is obtained for each RC-member. These capacitors neutralize the effect of the supply lead inductances and the stray capacitances of the individual resistors. However, the transfer bandwidth is still limited and, moreover, also dependent on the position of the RC-members 10 . . . 19 (referred to hereinafter as the measuring resistor arrangement) within the high-voltage generator. This is due to the stray capacitance between the individual RC-members and their surroundings, notably the grounded housing of the high-voltage generator; one of these capacitances is diagrammatically represented by a capacitor 9 which is shown in broken lines in the drawing. As is evident from the circuit diagram, in the case of quick variations of the high voltage, part of the current through the measuring resistor arrangement flows directly to ground via these stray capacitances. The currents flowing via the stray capacitances are larger as the high voltage is higher, so that the voltage drop occurring across the RC-member 19 at the high-voltage side is larger than that across the RC-member 10 which is connected to ground via the terminating RC-member 8. This limits the transfer bandwidth and reduces the measuring accuracy for high frequencies.